It has become increasingly desirable to reduce fuel consumption and improve overall SFC in a gas turbine. At the same time it is desirable to improve SFC without compromising on stall margin during all conditions of turbine engine operation.
Stall, or otherwise commonly known as surge, occurs when a compressor operating point/line rises to its stability limit that defines a limit between stable and unstable regions of flow. Below the limit, flow is stable, and a distance to the surge line, at a given flow rate, defines an amount of stall margin present at that given operating condition. The stability limit or surge line or stall line (depicted in FIG. 4 as 402) is the locus of the highest permissible stable pressure ratio (PRstall) at a given compressor inlet corrected flow, Wc, below which compressor operation is stable.
Gas turbines are traditionally operated having turbine cooling flow requirements established to keep the turbine metal temperatures at acceptable levels. This cooling flow, originating from the compressor, also maintains the stall margin requirements. At high power and during engine acceleration (including hot re-slam or bodie) where the stall margin requirements are high a certain level of cooling airflow is required that provides adequate protection against stall, whereas at part power (e.g., cruise) the cooling airflow requirements can be reduced to improve SFC.
In traditional engines the coolant or airflow requirements are established based on high power operating conditions where the turbine inlet temperature is high. And, in traditional engines cooling airflow is typically not reduced at lower power, even when the turbine inlet temperature is relatively low. Thus, for part power operation (e.g., cruise), as an example, excess cooling is provided adversely impacting SFC.
SFC on the other hand could be improved by reducing cooling flow during cruise conditions where the turbine temperatures are not high. However, coolant airflow based on cruising requirements would compromise stall margin at high power operation and during engine acceleration (including hot re-slam or bodie) if the amount of coolant airflow is not increased to the level existing at high power.
Balancing the desire to enhance SFC while not jeopardizing stall margin has become an increasing challenge to the gas turbine industry. Overcoming these concerns would be desirable and could save the industry substantial resources.